This invention relates generally to electronically controlled cooking appliances such as domestic ranges or cooktops featuring one or more automatic surface units which allow the user to select a Boil Mode to bring liquids contained in a utensil being heated by the automatic surface unit to a boil. More particularly this invention relates to a method and apparatus for predicting when the liquid being heated by the automatic surface unit begins to boil. This information can be used by the control to trigger a signal informing the user that boiling has begun. Also, the information could be used as an input to a variety of power control algorithms which could be used to vary the heating of the utensil for improved efficiency or to prevent boil over. An example of an electronically controlled cooking appliance to which the invention is particularly applicable is disclosed in commonly assigned U.S. Pat. No. 4,493,980 to Payne et al entitled "Power Control Arrangement for Automatic Surface Unit", which is hereby incorporated by reference.
U.S. Pat. No. 4,465,228 to Mori et al discloses one technique for implementing boil point detection in a cooking appliance. In accordance with Mori et al the gradient of the sensed utensil temperature is monitored and compared to a reference value to detect a decrease in gradient signifying that the water contained in the utensil has reached its boiling point. This gradient approach is based on the fact that the temperature of the water being heated in the utensil will not significantly exceed 100.degree. C. and consequently the sensed utensil temperature generally rises, if at all, at a much slower rate after boiling than prior to boiling. Upon detection of boiling, the heating level of the heater means is decreased to a level just sufficient to maintain boiling. Recognizing that the temperature gradient may vary greatly depending upon the size of the load being heated, and that for large loads the change in gradient may be difficult to detect and that for small loads, boil point detection may be late, the sampling times are varied as a function of the change in temperature since the next preceeding sample.
While generally speaking, the sensed temperature rises more slowly once the water in the utensil begins to boil, empirical observations have shown that for small water loads (less than 2 cups of water) the temperature sensed by the sensor may continue to increase for a significant period of time with little decrease in rate after the water has begun to boil. Similar results occur when utensils with warped or bowed bottom walls are employed. Under such operating conditions the gradient technique can be unreliable.
Another shortcoming of the gradient approach is its inability to detect the boil point if the water load is changed during the heat-up phase. It is not uncommon for the user to decide to add water to the utensil during the heat-up phase. The immediate affect of the addition of water is to lower the temperature of the load in the utensil and hence the temperature sensed by the sensor. If water is added early in the heat-up phase, the system may set up the wrong gradient for comparison purposes. If water is added relatively late in the heat-up phase, the resulting change in gradient may be incorrectly interpreted by the controller as the onset of boiling. Because the addition of water during the heat-up phase is a fairly common occurrence, it would be desirable to have a boil point detection arrangement which can reliably determine when boiling begins under such changing conditions.
It is therefore an object of the present invention to provide a relatively simple method and apparatus for reliably predicting when the boil point of water contained in a utensil being heated by an electronically controlled cooking appliance featuring an automatic surface unit will be reached regardless of the size of the water load.
It is a further object of the invention to provide a boil point prediction method and apparatus of the aforementioned type, the reliability of which is not affected by use of utensils in which the bottom wall is flat or bowed.
It is a further object of the invention to provide a boil point prediction method and apparatus of the aforementioned type, the reliability of which is not adversely affected by the addition of water during the heat-up phase.